6 Jan
2015
6 Jan
'15
6:33 p.m.
On 2015-01-06 17:56, Dan Cross wrote:
On Tue, Jan 6, 2015 at 11:51 AM, Johnny Billquist
<bqt(a)update.uu.se
<mailto:bqt@update.uu.se>> wrote:
On 2015-01-06 17:32, Mary Ann Horton<mah(a)mhorton.net
<mailto:mah@mhorton.net>> wrote:
Even with TERMCAP in the environment, there's still that quadratic
algorithm every time vi starts up.
I must be stupid or something. What quadratic algorithm?
vi gets the "correct" terminal database entry directly from the
environment. Admittedly, getting any variable out of the environment
means a linear search of the environment, but that's about it.
What am I missing? And once you have that, any operation still means
either searching through the terminal definition for the right
function, which in itself is also linear, unless you hash that up in
your program. But I fail to see where the quadratic behavior comes in.
I believe that Mary Ann is referring to repeatedly looking up
(presumably different) elements in the entry. Assuming that e.g. `vi`
looks up O(n) elements, where $n$ is the number of elements, doing a
linear scan for each, you'd end up with quadratic behavior.
Assuming that you'd look up all the elements of the termcap entry at
startup, and did each one from scratch, yes.
Hashing, or storing in some kind of balanced-tree like
structure or
something, would of course help but would also necessitate doing a copy
and would entail some additional memory inefficiency.
Hashing would indeed cause some extra memory, but not necessarily any
copying.
But that would beg the question, why is vi doing a repeated scan of the
terminal entry at startup, if not to find all the capabilities and store
this somewhere? And if doing a look for all of them, why not scan the
string from start to finish and store the information as it is found? At
which point we move from quadratic to linear time.
But now we're getting into the innards of vi, which I never looked at
anyway, and I guess is less relevant in this thread anyway.
The short of it (from what I got out of it) is that the move from
termcap to terminfo was mostly motivated by attribute name changing away
from fixed 2 character names.
A secondary motivation would be performance, but I don't really buy that
one. Since we only moved to terminfo on systems with plenty of memory,
performance of termcap could easily be on par anyway.
Thanks for the insights.
Johnny